Rock outcroppings will add a lot of detail to your landscape. The first picture shows a small piece of foam that I carved with my hacksaw blade. I began by painting it brown. While the brown paint was still wet I added a light tan to the high points and some black to the cracks. My brush was cleaned and dipped in water and gently brushed over the surface to blend and settle the colors into the crevices.
See if you can spot this styrofoam rock in the next steps. I'll use it near the tunnel.
As I shaped the tunnel I unintentionally formed a small knob. I decided to carve it into a rock outcropping. Keep an open mind and details like this just seem to happen.
Base Paste was brushed up to it and Grass Base applied.